Process of coating tungsten or molybdenum articles with precious metals.



F. A. FAHREN-WALD.

PROCESS 0F COATING TUNGSTEN 0R MOLYBDENUM ARTICLES WITH PRECIOUS METALS. APPLICATION FILE MAY 3|. Isle.

LQGLI w. Patnd Apr. 2, 1918'.

A wf

y awww/J5 consisting of tungsten, molybdenum,

FRANK A. FAHRENWALD, OF CLEVELAND, OHIO, DEDICATED, BY MESN E ASSIGNIVIEN'JUS,E

TO THE GOVERNMENT 0F THE UNITED 0F THE UNITED STATES l0F AMERICA..

STATES 0F AMERICA AND T0 THE PEOPLE rEooEss oF coATINe TUNGSTEN oRi/ionYBnEnUM ARTICLES WITH. PRECIOUS METALS.

Applicationledv May 31,1916. Serial No. 100,997.

Specification of Letters Patent.

Patented Apr. 2, 1913.

(DEDICATED T0 THE PUBLIC.)

To all whom t may concern:

Beit known that I, FRANK A. FAHREN- WALD, a citizen of the United States, residing at Cleveland in the county of Cuyahoga and State of 0h10, have 'invented a certain new and useful Improvement in Processes of Coating Tungsten or Molybdenum Articles with Precious Metals, of which the follow-` ing is a full, clear, and exact drawings.

This application is made under the act of March 3, 1883, chapter 14:3 (22 Stat., 625) and the invention herein described and claimed may be used by the Government of the. United States or 'its officers or employees in the prosecution of work for the Government or any person in the .United States without the payment of any royalty thereon.

This invention relates to a.4 metal body or other oxidizable metal, and a layer of precious or non-oxidizable metal intimately connected therewith so as to -permit the soldering, or brazing ofthe article, or other high temperature working thereof without the occurrence of oxidation.

Tungsten, particularly in the malleable and ductile states described in the patent of William D. Coolidge, No. 1,082,933, dated December 30, 1913, is a most extraordinary metal by reason of its rigidity, hardness, tensile strength, insolubility, high melting point and other physical and chemical properties. Furthermore it has this peculiarty, that any desired manipulation may be performed upon it whatsoever without drawing `its temper or changing its state, provided the temperature to which it is subjected be maintained lower than the critical point of its creation, which my experiments indicate molybdenum described in myapplication description,y reference being had to /the accompanying filed J'une 3, 1916, Serial N o. 101,502. However a disadvantage has been that it oxidizes very easily upon its surface at any temperatures higher than a dull red (say 600 C.) and,owing to this property and to the small aHnity which it possesses for different soldering elements, it was for along time found impossible to weld, solder, braze, or otherwise unite different parts of these metals. These two drawbacks, of easy oxidation and diiiicult soldering, have constituted the only impediments t0 the extensive use of these metals in dentistry and in many other practical arts, as well as in numberless scientific and philosophic uses.

The first solution attempted lay in searching for alloys which would preserve the beneficial `properties of the material while avoiding its harmful properties, and to this end various attempts were made to secure an lalloy of tungsten 1n connection with gold,

platinum, palladium and the like noble metals, but in most cases the alinity of tungstenv for such other metals is so small and the diiierence between their melting points so greatl that no alloy could be secured. However it was discovered that while the large pieces of tungsten were not appreciably dissolved they were nevertheless removed rom the molten bath with a beautiful impervious coating. As either of these furnished the surface qualities which tungsten lacks the two remaining conditions seemed to be fulilled. And so they were; but after this bath'the tungsten wires were as ,brittle as glass. The problem thus arose of applying a coating of such material and properties of the tungsten would not .be destroyed during the process. The crystallizapropei' composition of the coating material,

01. improper time element.

'under such conditions that the other valuableY tion Aof the tungsten was obviously due to to be somewhere between-1400 and 1500 C.' f The same properties are present in evenvv higher degree in the alloy of tungsten and 9 A short resumeJ of my results is given in thatonly the metals silver, gold and palladithe following table, since it was discovered um oered promise of complete success.

Experiments in coating tungsten with gold, silver, and palladium, under varying conditions.

. Temperature oi bath.

Composition.

.Time 1, 100 C. 1,350 CL. 1,500 C. 1,600 C.

Ag. Au. Pd. tEuegcsttg Coating. ,Fu Coating. Coating gj Coating.

Perct.

100. 10 None... Very poor. None Very poor. Very brittle.... Poor.... Very brittle. Poor. 100 None.... Very poor. Quite brittle. Very poor. Verybrittle.... Poor.... Verybrittle. Poor.

None... Good...... None Good..... Slightlybrittle. Good.... Verybrittle. Good. 'None.... Good..... Quitebrittle... Good.... Verybrittle.,Good. Verybrittle. Good. Verybritt1e.... Good.... Very brittle. Good. Very brittle.... Good.... Verybrittle. Good. Very brittle.... Good.... Verybrittle. Good. Very brittle.... Good.... Verybrittle. Good. Verybrittle.... Good. Very brittle. Good. Verybrittle.... Good.... Verybrittle...........

and became brittle at 1,700" C. in from 10 to 20 mm.

5 Silver alone may be employed but adheres rather weakly and appears for some unknown reason to accelerate crystallization in the tungsten, wherefore its use alone is Y not recommended. Platinum alone, certain other metals such as iridium, have a marked dissolving effect, and some ofthem therefore adhere very tenaciously to the' surface of the tungsten. However platinum requires very delicate manipula tion since its melting temperature is dangerously close to the equiaxing temperature of the tungsten. Palladium alone adheres vvery strongly to and even alloys slightly with the tungsten, but also, when used alone, appears to accelerate crystallization, though when used in company with a preponder ance of gold this deteriorating effect is not observable.- Gold alone has comparatively little aliinity for tungsten, and the line of ,'25A contact is rather clearly defined, though it adheres reasonably well for all practical purposes and Lforms a most suitable coating without any injury to the' metal beneath; but by adding small amounts of palladium f to the lcoating bath its melting point is raised, and, at the same time, a better bond 'between the tungsten and this protective layer is obtained. 'It will be noted from the foregoingtable, that with a preponderance of palladium in the vcoating bath the tungsten came out very brittle, but'that-when the palladium content wasreduced to ve or ten, per cent. the bath could be reduced in temperature to 1350 C., and no eii'ect was t'produced upon the tungsten within the length of time necessary eration.

The time and temperature constants ap.V pear to be interdependent, the ductile tungt5 sten undergomg a crystalline rearrangement for the coating opand maximum quantities ids of the this bath the metal matically acquired.

operation dizing.

temperature.

These tests were made on drawn wires of 30-mi1 size, which without the coating, and heated in an atmosphere of hydrogen, crystallized either upon a short exposure to a high temperature or a longer exposure to a lower Thus I have found, that a subjection of the ductile tungsten to a temperature of 14:00o C. or even 1450o for five or ten secondswill ordinarily not injure the metal, though I consider ten seconds at 141000 C. or thirty seconds at 1350o C. the

that should be considered for practical shop or technical uses.

I\-have discovered that thiscoa'ting process may be performed in the open atmos- I phere by the use of a suitable flux. In small the tungsten articles my be dipped into a bath of fused sodium carbonate, sodium borate, or other oxygen alkali salt which will combine withl the superficial oxtungsten to form a suitable tungstate which is soluble in the bath. vThe tungsten article can then be removed from with a cleansing and protective" shell of linx adhering to it, and dropped into bath whereupon the flux is autoremoved and the metal coating For the larger scale treatment of tungsten wire I have supported the wire in such a way that a bight or downwardly extending loop ofthe wire should di beneath the surface of fused alkali 'ina ceptacle and passed through a molten button of coating metal locatedat the bottom thereof. The tungsten wire being drawn uniformly throughthis bath, the coating aring rebecome continuous, the portions iof the wire outside the bath being, of course, maintained suiliciently cool to lprevent oxi- Either of these processes results ina wire or article having a coating of gold or other precious metal -thereon of considerable 85 thickness, owing to the adhesion of the-gold.

However if the time and temperature elements above mentioned are kept within the limits indicated, the strength, rigidity, ductility and tensile strength of the tungsten will not be in any way impaired, but the metal becomes proof against oxidation andY can be Asoldered with the greatest ease, wherefore the resulting article will be found suited for uses which have heretofore been f satisfied only by such expensive materials as platinum and platinum iridium alloys.

In the drawings accompanying and vforming a -part of this application I have illustrated various manipulations for eiiecting this coating operation, some of the same being confined to use with wire, and others being susceptible equally for the coating of other shapes or articles. In these drawings Figure 1 represents the simplest operation, merely of dipping in an open Crucible under the surface .of a flux; Fig. 2 represents the drawing of a wire through a perforated crucible; Fig. 3 represents the drawing of 4 represents' the flux. `5 represents the article to be coated, here shown as a dental pin held in suitable ,tongs 6, the tongs preferably having porcelain or the like graspin points in order that the distribution of t e gold upon the article may not be aected thereby.

Withthe ordinary Bunsen burner or with the ordinary blast lamp, unless very, powerfully urged,`there-` is no danger of obtaining a temperature so high as to equlaxiate the tungsten. It is obvious that disks, cylinders or other shapes could equally well be coated as the wire or rod shown.

In Fig. 2 I have illustrated an arrangement whereby a wire can be coated by a continuous operation. In this view the crucible 10 is illustrated as formed with an aperture 11 in its lower part closely receiving the wire 12. i The mass 13 ofthe desired coating metal is fused'within the crucible in any suitable manner, preferably beneath the surface of va flux 14, although in this case I do not consider the flux indispensable. The wirel`1 2 being drawn steadily upwardly at the requisite speed, which can be-easily ascertained v by observation for a few seconds, a perfect coating of metal will bedeposited thereon. By suitably arranging the heating appliance (not shown) and by mechanically scraping or cleaning the wire before drawing theA f same into the metal, the coatingwill adhere suciently without other precautions. The coating ofl metal upon the wire after it emerges above the molten metal will sufficiently prevent any oxidation or other change in the same, although a flux of some nature is frequently desirable to increase the iiuidity of the metal and to restrain its cooling. v p

In Fig.v 3 I have illustrated another expedient for the continuous coating of a wire. In this view I have illustrated an electrical resistance furnace comprising a carbon rod 20, suitably yhollowed out or recessedat 21 and traversed by a porcelain or other suitable stud 22. Suitable metallic terminals 2,3-23 are clamped against thevends of the carbon rod and provided with electrical energy by the cables 24e-24. 'Ihe heat produced in the carbon melts the gold or other coating metal illustrated at25, and also the flux 26overlying the same, so that a wire 27 introduced beneath the stud 22 and pulledI in either direction will become cleansed and coated in a single operation.

In Fig. 4 I have illustrated an electrical resistance furnace comprising resistanceVJ wires 30 coiled about the lexterior of a tube 31, said tube having at one end the closure 32 connected with the pipe 33 through which some inert gas like nitrogen or some reducing gas like hydrogen can be forced. The gold or other coating metal is shown at 34 contained in a boat 35 and the article 36 to 'be coated is handled by means of the tongs 37, a suitable shield 38 being employed if desired. It will be understood that the above are only a very few of the arrangements and manipulations which could be I employed to the end in view.

While I have described my improved process chiefly in connection with tungsten,

it is to be understood that the word molybdenum could be substituted in this specification `wherever the word tungsten has been used,

since the properties of the two metals are very similar, and I ylikewise consider the alloy of tungsten and molybdenum as comprised within the meaning of this specication equally with the metallic tungsten.

Having thus described my invention, what g I claim is: Y l

1. In the process of soldering oxidizable metals like tungsten or molybdenum or their alloys, the step which consists in rst coating said oxidizable metal with a non-oxidizable metal as gold, silver, platinum, palladium or an alloy thereof.

2. rIhe process of coating tungsten and 'molybdenum with a precious metal, as gold,

which consists in applying the coating metal in molten form in i connection with a flux containing the oxygen salt of an alkali metal.

3. The process of preparing articles of ductile tungsten and molybdenum for soldering or brazing which consists in heating such articles for not longer than about'30 seconds to ya temperature Iof not more than about 1350o C. in contact with molten gold and a flux containing the oxygen salt of an alkali metal. v

4:. The process of coating ductile tungsten with gold which consists in rst immersing said tungsten in a bath containing the fused oxygen salt of an alkali metal, and, second, immersing said article for not more than about 30 seconds in a bath containing principally or entirely molten gold at a temperature of not more than about 1350o C.

5. The process of coating tungsten or molybdenum Wires With a non-oxidizable metallic coating such as gold, silver, platinum or palladium, which consists in drawing said Wires irst through a bath containing the molten-oxygen salt of an alkali metal and thereafter through a bath of the molten coating metal maintained at a temperature not higher than about 1400o C.

6. The process of treating tungsten or molybdenum, comprising using as a flux a mixture consisting of or containing the oxygen salt of an alkali metal substantially as specified.

7. The process of treating tungsten or molybdenum, consisting in coating it with a compound containing gold and palladium.

In testimony whereof, I hereunto aiix my signature.

FRANK AFAHRENWALD. 

